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EVE-280 cells should these be clamped tight or spaced for expansion?

Oh ..... @Dzl .... excellent work for the data mining you have been doing for this and other threads.

.... and @ghostwriter66 .... Excellent work for actually getting EVE engineers to talk to you.
 
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@ghostwriter66 One question I would have is how many charge cycles with no compression would be required to reduce the cycle life of the cells significantly .... does even a few cycles with no compression reduce the cell life or would it be spread over the life of the cells.

I guess what I am wondering is how much damage can be done when initially capacity testing individual cells, or top balancing a pack with no compression fixture.
 
I guess what I am wondering is how much damage can be done when initially capacity testing individual cells, or top balancing a pack with no compression fixture.

Minimal, especially if we are talking low C rate. This type of thing is cumulative. The miniscule movement between charge and discharge causes the capacity loss over time. Its likely exacerbated by high C rates or vibration. I am almost certain that EVE doesn't use such a fixture for initial charging/testing on each cell.
 
Here's another option on springs from my post back in May...
https://www.leespring.com/compression-springs-hefty?search=LHL625D

I did buy some but never did install. Maybe I'll try them now.
Which one of those did you get? Did you have any problems buying from them direct? How much did they cost?

I was looking for an online seller. There is a vendor on Ebay that is selling the Danly springs (toolandequipsales) and I was considering sending them a message to see if they could get the ones I was looking for.
 
Which one of those did you get? Did you have any problems buying from them direct? How much did they cost?

I was looking for an online seller. There is a vendor on Ebay that is selling the Danly springs (toolandequipsales) and I was considering sending them a message to see if they could get the ones I was looking for.
I ordered online from Lee Springs. After looking at spring rates, I ordered two different ones to try:
LHL 625D 03, $3.97 each
LHL 500D 03, $3.40 each

You have to balance having enough pressure to get desired compression against having enough travel to accomodate any expansion/contraction. These two seemed in the right range to me.
 
@Airtime .... AHHH .... I see now that if you click on it there is an option to add it to the cart. Please let us know how it works out .... Pics even better.
 
OK ... talked to the senior design architect for EVE. He was very specific and short response. So these are NOT all his answers since I don't want to ramble BUT these are the ones that I think are important to us. Let me preface that he started out that although there are straightforward chemistry (liFePO4) involved -- there is still allot of mystery when it comes to LFP and why it does certain things a certain way ...

SOOOO - he said - if he was a DIY'r and doing this for an off-grid cabin or RV he would do the battery virtually the same way that the junior engineers said last night. (see my previous posts earlier) (post #215)

Any compression is better than no compression. BUT NO compression is better than compressing it above 18 PSI.

Anywhere between 6-17 are good numbers - but those numbers will change on SOC and temp.

SOOO In a perfect world - when your SOC is at 50% - your PSI would be 12 and your temp is 72. He would do the adjustments at 50% SOC and then assume that 100% SOC will be a few PSI higher and 0% a few pounds lower. As long as your high and low is between 17-6 you are fine but a constant 12 is what you are looking for.

For temp - and I know this was not one of the questions -- BUT 72 is perfect -- and actually its better to be at 52 than 82 .. although 72 is perfect - when temp starts going above 72, the UNHAPPY FACE scale starts going up exponentially as the temp climbs and when the temp lowers its more linear ... (if that makes sense). That also will affect the PSI.

In fact nearly everything affects the PSI -- and over the life of a LFP you will get a couple more years from a battery at 12PSI CONSTANTLY then one going between 6-17 -- BUT that's NOT the real world unless you have a spring or mechanical controlled clamping system.

BUT technically 12 is the magic number and if you CAN KEEP it at 12 then you will probably get ALMOST 1.6X the cycles out of these batteries as you would if you do nothing.

One of the guys had a question -- ans.. Keeping it snug in a metal box at 0% PSI and then as the expeand the PSI gets higher is still better than nothing - but only marginally.

If I missed something let me know and its probably in my notes ... he really was excited that someone from the states was actually interested in this ...
 
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The springs discussed should work good. There are several small (less than 1" square) coil springs available Choose a reasonable rate, say 100-400lb per inch. Then just compress the springs as needed with threaded rod. If you have 4 rods and 200lb/in springs, compress each spring by 0.825". Obviously choose a spring that wont go solid before you reach the desired clamping force.
 
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SOOOOO - i guess the next thread is - WHO HAS A GREAT IDEA HOW TO BUILD A SPRING LOADED 12PSI storage capability ???

So when I hear PSI I think tire pressure.

How bout a battery box with an air bladder in between the cells and the rigid box. Inflate to 12 PSI easy peasy.

The right sized innertube might even work?

I have no idea if that is thinking inside or outside of the box ?
 
An air bladder would be too sensitive to changes in ambient temperature I think.

For sourcing springs, mcmaster carr has a decent selection, though it can be hard to navigate filter at times.
 
Well a simple tire gauge can solve that. Probably can find a Bluetooth one.

The bladder would need adjusted fairly often if the temperatures swing much at all. Plus you would need to check regularly for leaks anyways.

The coil spring method is one and done, and probably wont cost any more.
 
I have a stupid question. I have six 5/16 threaded rods and six 5/16 stainless steel lock washers. If I torque all six lock washers down until they are flat, approximately how much PSI would that be? Enough? If too much I can use 4 lock washers. I have built a 8s square pack but still need to mount the plywood. I am not going to worry about springs and what not. Even with 2000 cycles the cells will most likely outlive me.
 
To further elaborate on the spring selection. You first determine how many rods you want/need. Lets say 4.

Then you determine the clamping load. If you have one cell stack, that's about 600lb 600/4 =150lb per spring.

Now we need a coil spring which can apply 150lb before it binds/goes solid. Additionally we want a spring that won't require use to tighten a nut down 3" to get that force. We also want a spring with a small enough OD/ID that it can be used with a threaded rod and thick washer.

For example a hypothetical .8" OD spring with a spring rate of 200lb/in. For ever inch its compressed, it requires 200lbs of force. This spring has a max load of 175lb, which is fine for this example.

To get 150lb we then need to compress the spring by 200/150=1.33".

From there its simply a matter of measuring the springs free length, and then tightening each nut down until the springs are 1.33" shorter.
 
I have a stupid question. I have six 5/16 threaded rods and six 5/16 stainless steel lock washers. If I torque all six lock washers down until they are flat, approximately how much PSI would that be? Enough? If too much I can use 4 lock washers. I have built a 8s square pack but still need to mount the plywood. I am not going to worry about springs and what not. Even with 2000 cycles the cells will most likely outlive me.

There is lots of variance, but it can take 15-40lbs to compress a spring washer flat. I don't really call them lockwashers, as the spring type don't really improve fastener retention in most cases.
 
There is lots of variance, but it can take 15-40lbs to compress a spring washer flat. I don't really call them lockwashers, as the spring type don't really improve fastener retention in most cases.
Thanks. So considering EVE is calling for 12PSI it should be ok to use all 6.
 
Thanks. So considering EVE is calling for 12PSI it should be ok to use all 6.
Sure, you can always do a quick and dirty test by hand. Push on a washer with a deep socket, and guess how much weight you are putting on it.
 
I strongly dislike mechanical engineering - and to me thats what these spring things are -- i can usually finally figure it out but always wonder if I actually got it right our not ...

So if you were me - and you were looking at the below PDF -- and I am assuming that it would be the RED section on page 14 .. and you had (4) 3/8 threaded bolts you were using to press aluminium plates against the battery -- which one of these things should I order to get the right 12PSI (600kPF) compression that I am looking for ?? ...


Thx
 
I strongly dislike mechanical engineering - and to me thats what these spring things are -- i can usually finally figure it out but always wonder if I actually got it right our not ...

So if you were me - and you were looking at the below PDF -- and I am assuming that it would be the RED section on page 14 .. and you had (4) 3/8 threaded bolts you were using to press aluminium plates against the battery -- which one of these things should I order to get the right 12PSI (600kPF) compression that I am looking for ?? ...


Thx
I share this sentiment.

A nice clear explainer/primer from someone that feels they know what they are doing would be great!
 
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I am a LONG ways from being a mechanical engineer, but maybe one will check in.

Here is my theory.
Assuming we are in agreement that the desired pressure is 12 PSI,
The first step would be to determine the square inches for the side of the battery. For my 7" x 8" battery that would be 56 Sq inches.
Second, Multiply that by 12 to get the desired total pressure. For mine that is 56 x 12 = 672 lbs force.
Third, divide 672 by the number of threaded rods to get the pressure / rod. 672 / 4 = 168 lbs per rod.
Fourth ... convert that to KG. So we need a spring with 76.2 KG at a reasonable compression rate.

So .... for my batteries it looks like the 93-2030 on page 15 might be a good fit. (They show how much to compress it for 3 different pressure on the right side of the chart. Every spring in that group has the same capability but with different lengths.
 
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I am a LONG ways from being a mechanical engineer, but maybe one will check in.

Here is my theory.
Assuming we are in agreement that the desired pressure is 12 PSI,
The first step would be to determine the square inches for the side of the battery. For my 7" x 8" battery that would be 56 Sq inches.
Second, Multiply that by 12 to get the desired total pressure. For mine that is 56 x 12 = 672 lbs force.
Third, divide 672 by the number of threaded rods to get the pressure / rod. 672 / 4 = 168 lbs per rod.
Fourth ... convert that to KG. So we need a spring with 76.2 KG at a reasonable compression rate.
I think most of the middle steps can be skipped because EVE has done the math for us already (300KGF),
I originally got the 12PSI by doing the math from KGF to PSI.
So we don't need to convert back, we can just start with the original spec.

But I think your overall explanation is correct in principle, divide KGF by number of rods. And then choose springs to match whatever you come up with.

Where I personally start feeling less sure of myself is how to choose the spring and how to implement the design / how to know if the pressure is in the right ballpark. Is it all about measuring the compression of the springs?

edit: i'll checkout the page you mentioned, that sounds like just what i was hoping for.
 
I think most of the middle steps can be skipped because EVE has done the math for us (300KGF),
I originally got the 12PSI by doing the math from KGF to PSI.
So we don't need to convert back, we can just start with the original spec.

But I think your overall explanation is correct in principle, divide KGF by number of rods. And then choose springs to match whatever you come up with.

Where I personally start feeling less sure of myself is how to choose the spring and how to implement the design / how to know if the pressure is in the right ballpark. Is it all about measuring the compression of the springs?
Yeah ... It's more of a universal compression calculation thing .... My cells aren't actually EVE cells. Sounds like 12 psi compression might be good for all of the prismatic cells. I don't know what those cells of Dacian's are.
 

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